Add rustdoc test for double-hyphen to dash doc comment conversion

Author: GuillaumeGomez

tests/rustdoc/double-hyphen-to-dash.rs

@@ -0,0 +1,9 @@
+// This test ensures that `--` (double-hyphen) is correctly converted into `–` (dash).
+
+#![crate_name = "foo"]
+
+// @has 'foo/index.html' '//*[@class="desc docblock-short"]' '–'
+// @has 'foo/struct.Bar.html' '//*[@class="docblock"]' '–'
+
+/// --
+pub struct Bar;